Mercury and other heavy metals are contained in addition to nitrogen oxides and sulfur oxides in an exhaust gas discharged from a thermal power plant boiler, etc., which is a combustion apparatus that uses coal or other fossil fuel. Although the nitrogen oxides are removed by NOx removal equipment and the sulfur oxides are removed by a desulfurizer, mercury and other heavy metals cannot be removed by the NOx removal equipment or desulfurizer and cannot be trapped completely by a precipitator for removing soot/dust in the exhaust gas. Mercury and other heavy metals are high in toxicity, and thus emission restrictions thereof have recently become stricter and methods for removing mercury and other heavy metals are being examined.
FIG. 6 is an overall system diagram of a conventional purifying system for the exhaust gas discharged from the abovementioned combustion apparatus. The exhaust gas 2 discharged from the boiler 1 is introduced into NOx removal equipment 3 for removal of the nitrogen oxides (NOx) in the exhaust gas and thereafter heats combustion air 5 for the boiler 1 in an air preheater 4 by heat exchange. A large portion of soot/dust in the exhaust gas is then removed by a dry electric precipitator 6, and the exhaust gas is then raised in pressure and introduced into a wet desulfurizer 8 by an induction fan 7. Sulfur oxide (SO2) in the exhaust gas is removed by gas-liquid contact from the exhaust gas 2 introduced into the wet desulfurizer 8. The exhaust gas 2, which is cooled to a saturation gas temperature in the wet desulfurizer 8, is removed of sulfur trioxide (SO3) mist by a wet electric precipitator 9 and then discharged into air from a chimney 10.
As mentioned above, environmental pollutants contained in the exhaust gas from the boiler 1 that uses coal, etc., as fuel include the sulfur oxides (SO2, SO3), nitrogen oxides (NOx), soot/dust, and mercury (Hg) and other heavy metals, and as purifying equipment, the NOx removal equipment 3 is installed for the NOx, the desulfurizer 8 is installed for sulfur dioxide (SO2), the dry electric precipitator 6 and the desulfurizer 8 are installed for soot/dust, and the wet electric precipitator 9 is installed for sulfur trioxide (SO3).
Also, it is known in regard to sulfur trioxide (SO3) that when a basic substance, containing a sodium component, etc., is blown into the exhaust gas from basic substance injection systems 11 at an upstream side of the NOx removal equipment 3, an upstream side of the air preheater 4, and an upstream side of the dry electric precipitator 6, etc., at a temperature higher than a dew point of sulfur trioxide (SO3) (approximately 130° C. or higher) as shown in FIG. 7, conversion of SO3 to SO2 occurs. In a case where a concentration of sulfur trioxide (SO3) is high and yet an environmental regulation value (legally designated value or voluntary regulation value) is stringent, not only the wet electric precipitator 9 but the above-described method is also adopted in combination in many cases to eliminate sulfur trioxide (SO3) efficiently (for example, Japanese Published Unexamined Patent Application No. 2006-326575).
Meanwhile, generally for Hg and other heavy metals, activated carbon is sprayed from an activated carbon injection system 12 as shown in FIG. 8 to make the Hg and other heavy metals to be adsorbed onto the activated carbon surface, and the Hg and other heavy metals are removed along with the activated carbon by the dry electric precipitator 6.
The exhaust gas purifying systems in the overall system diagrams of FIGS. 7 and 8 have configurations with which the basic substance injection systems 11 and the activated carbon injection system 12 are respectively added to the system diagram described in FIG. 6.    [Patent Document 1] Japanese Published Unexamined Patent Application No. 2006-326575